Frequency splitting and independent limitation of vehicle torque control

ABSTRACT

A method for optimizing torque control in a vehicle having a controller and a rotating member includes generating a closed-loop total proportional torque command using a state space feedback portion of the controller, and splitting the total proportional torque command into high-frequency and low-frequency proportional torque components. A total proportional torque is passed to the rotating member to provide driveline damping control when speed control is not required. The high-frequency proportional torque component is passed to the rotating member to provide driveline damping control, and the low-frequency torque component is passed with a total integral torque command to the rotating member to provide speed control, when speed control is required. A vehicle includes a controller having proportional-integral control capabilities and a state space observer, and a powertrain having a rotating member whose speed and damping characteristics are controlled by the controller.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/382,515, which was filed on Sep. 14, 2010, and whichis hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to the splitting of a total proportionaltorque command from a state observer-based control law in a vehicle intodifferent high-frequency and low-frequency torque components, and theindependent limitation of damping and speed control torques using thesetorque components.

BACKGROUND

Certain vehicles can be powered using one or more high-voltage electrictraction motors. Hybrid electric vehicles (HEV) having a full hybridpowertrain selectively use an internal combustion engine, either aloneor in combination with the traction motor(s). A battery electric vehicle(BEV) uses a traction motor as the sole power source, while anextended-range electric vehicle (EREV) uses a gas engine to power agenerator when additional electrical energy is required. Typically, afull HEV runs in an electric vehicle (EV) operating mode up to athreshold vehicle speed, and then automatically starts the engine uponreaching the threshold speed. Thereafter, the HEV transitions at leastpartially to engine torque.

The speeds of the various rotating members of an HEV, BEV, EREV, or EVpowertrain, for instance an input speed to a transmission input memberand/or a clutch slip speed of one or more of the clutches used within atransmission, may be controlled using a proportional-integral (PI) or aproportional-integral-derivative (PID)-capable controller. Different PIor PID controllers may be used to govern a speed of a given rotatingmember, as well as to damp any driveline oscillations or pulsations. Astate observer can be used as part of the overall control law to providestate estimation within a particular physical system, e.g., thetransmission, using various input and output parameters, as well as anyrequired linear or other suitable state equations. In some vehicles, acommon state space control law may be applied to both the speed controltorque and the driveline damping control torque.

SUMMARY

Accordingly, a method is disclosed herein for splitting a totalproportional motor torque command from a state space observer-basedcontrol law in a vehicle having at least one electric traction motor.The total proportional torque command is generated along with a totalintegral torque command by a proportional-integral (PI) or aproportional-integral-derivative (PID) controller, as are wellunderstood in the art. The term “frequency splitting” as used hereinrefers to the selective separation, using filtering or other suitablemeans, of the total proportional torque command into separatehigh-frequency and low-frequency torque components when speed control isrequired.

Once separated, the high-frequency torque component may be assigned alower priority, and used as the damping control torque command whenspeed control is required, i.e., when one or more speed degrees offreedom are present in the system being controlled. The low-frequencytorque component may be added to the integral torque, i.e., the torqueoutput of an integrator portion of the controller, and passed as thespeed control torque command whenever speed control is required. Whenspeed control is not required, the total proportional torque command maybe passed through a damping control path to the rotating member and usedto control driveline damping. After frequency splitting, the separatetorque components can be independently gain-limited with respect to eachother as needed. Such a result may be useful when faced with apredetermined constraint such as a battery power limit or a tractionmotor torque limit.

The controller disclosed herein selectively passes the low-frequencytorque component of the total proportional torque command through acommon speed control path with the integral torque command, and uses thecombined torque to provide speed control over the rotating member asneeded. This may be done via a torque determining module or othersuitable algorithm or software subroutine. A calibrated low-pass filtercan be used to isolate the desired low-frequency torque component, withan optional software trigger used to determine when such filtering isrequired. The high-frequency torque component may be calculated bysubtracting the low-frequency torque component from the totalproportional torque command. The high-frequency torque component canthen be used as the driveline damping torque, thus stabilizing instanceswhere damping torque would otherwise be clipped or temporarily limited,e.g., due to the presence of predetermined constraints. Thelow-frequency torque and the total integral torque are passed even whenthe high-frequency torque component is limited.

In particular, the present method optimizes torque control in a vehiclehaving a controller and a powertrain with a rotating member. Thecontroller selectively combines an integral torque and a low-frequencyproportional torque component to provide speed control over the rotatingmember, with driveline damping control provided via a high-frequencyproportional torque component when speed control is required. The methodincludes selectively splitting a total proportional torque command intohigh-frequency and low-frequency torque components when speed control isrequired, and then passing the combined low-frequency torque componentand a total integral torque to the rotating member through a commonspeed control path. This optimizes control situations where a batterypower constraint, a clutch torque constraint, a motor torque constraint,or other predetermined constraint is present.

A vehicle comprising a controller having proportional-integral controlcapabilities and a state space observer, and a powertrain having arotating member whose speed and damping characteristics are controlledby the controller. The controller is configured for executing the abovedescribed method.

The above features and advantages and other features and advantages ofthe present invention are readily apparent from the following detaileddescription of the best modes for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a vehicle having a controller withstate observer and proportional-integral (PI) control capabilities asdisclosed herein;

FIG. 2 is a schematic illustration of the torque signal splittingfunctionality of the controller shown in FIG. 1; and

FIG. 3 is a flow chart describing a control method usable with thevehicle shown in FIG. 1.

DESCRIPTION

Referring to the drawings, a vehicle 10 is shown schematically inFIG. 1. The vehicle 10 may be propelled at least part of the time in anelectric vehicle (EV) propulsion mode, e.g., as a hybrid electricvehicle as shown, or as a battery electric or extended-range electricvehicle. The vehicle 10 includes a controller 11 that providesproportional-integral (PI) functionality via a PI algorithm 80 and statespace feedback control via a state observer 90, to thereby control speedtorque and driveline damping-related torque in the vehicle 10. The stateobserver 90 provides state space representation in the form of amathematical model or models of the various systems being controlled,with the models forming a related set of inputs, outputs, and statevariables as understood in the art. PI functionality of the PI algorithm80 is described in detail below.

As explained below with reference to FIGS. 2 and 3, the controller 11executes the present method 100 when speed control is required in orderto selectively split a total proportional torque command generated bythe PI algorithm 80, i.e., the proportional (P) terms of a PI controllaw, into a high-frequency proportional torque component and alow-frequency proportional torque component. The controller 11 combinesspeed control, i.e., the low-frequency proportional torque component andthe integral (I) terms of the PI control law, with driveline dampingcontrol, i.e., the high-frequency proportional torque component of thesame PI control law. The integral component and low-frequencyproportional torque component are passed to a torquedetermination/arbitration algorithm or subroutine 66 (see FIG. 2) thatis resident within or otherwise executable by the controller 11.

As used herein, the terms “high-frequency” and “low-frequency” aredetermined with respect to error calculation gains. Proportional torqueis generated as a function of any gained error in the multiple estimatedstates and reference or target states generated by the controller 11.For example, at time (t), the torque based on the gained error may jumpfrom 0 Nm to 10 Nm. A sudden jump of this magnitude in one processorloop may be considered high-frequency. Such a loop may be approximately6.25 ms in one possible embodiment, although the loop time will varywith the vehicle design. If the torque remained at 10 Nm for apredetermined number of subsequent processor loops, this may beconsidered low-frequency, i.e., the terms “high” and “low” are relativeto the processor loop time as well as to each other.

Controlling to a speed target typically requires a slower response timethan controlling to a damping target. Therefore, in one embodiment thecontroller 11 can use a calibrated filtering frequency, e.g.,approximately 2 Hz, so that a torque falling below that level is usedfor speed control and a torque falling above that level is used fordriveline damping. The calibrated filtering frequency value can bemodified as needed during the various operating states of the vehicle10, and based on the particular frequency response of the driveline.

Still referring to FIG. 1, the controller 11 can initially prioritizetorque commands, for example in the following order: (1) speed control,which is provided by the combined integral torque component andlow-frequency proportional torque component; (2) propulsion torque,i.e., transmission output torque; and (3) driveline damping torque,which is at least the high-frequency proportional torque component. Thecontroller 11 can determine whether battery power limits, motor torquelimits, or another predetermined constraint is present. The torquecomponent having the lowest priority is temporarily clipped or otherwiselimited during such a constraint. This is the driveline damping torqueor high-frequency proportional torque. However, it is observed hereinthat passing only the total integral torque without the proportionaltorque component can result in system instability under certainoperating conditions. The controller 11 and the present method 100 aretherefore structured to address this issue.

When configured as a hybrid electric vehicle as shown in FIG. 1, aninternal combustion engine 12 can be selectively connected to atransmission 14 via an input clutch and damper assembly 15. The clutchand damper assembly 15 is operable for damping transient pulsations fromthe connection between a rotating crankshaft 13 of engine 12 and aninput shaft 17 of the transmission 14. High-voltage electrical tractionmotors 16, 116 selectively deliver motor torque to the transmission 14as needed, and thereby power the vehicle 10 in an EV propulsion mode.This may occur up to a threshold vehicle speed. Above the thresholdspeed, the engine 12 can be automatically restarted, and engine outputtorque thereafter can be used to power the input shaft 17.

The transmission 14 has an output shaft 19 connected to a set of drivewheels 20. The transmission 14 may be configured as anelectrically-variable transmission (EVT) or any other suitabletransmission capable of transmitting torque to the wheels 20 via theoutput shaft 19. The output shaft 19 delivers the output torque (arrow33) in response to a torque request from a driver of vehicle 10, e.g., adepression of an accelerator pedal.

The traction motors 16, 116 may be configured in one possible embodimentas a multi-phase electric machine of approximately 60 VAC toapproximately 300 VAC or more depending on the required design. Otherembodiments may be used, e.g., induction motors, depending on thevehicle design. Each traction motor 16, 116 is electrically connected toan energy storage system (ESS) 26 via a high-voltage DC bus, a powerinverter module 25, and a high-voltage AC bus. A DC-DC converter (notshown) may be used to regulate the voltage to a 12 VDC auxiliary powersystem aboard the vehicle.

The method 100 can be programmed as a computer-executable set ofinstructions or code, and stored on a tangible/non-transitorycomputer-readable medium or distributed media. Such instructions or codecan then be selectively executed by associated hardware components ofthe controller 11, e.g., a host machine or computer device configured asset forth below. The controller 11 may be a single control device or adistributed networked control device that is electrically connected toor otherwise placed in electrical communication with the engine 12, thetraction motors 16 and 116, and the transmission 14 via suitable controlchannels. Such control channels may include any required transferconductors providing a hard-wired or wireless control link suitable fortransmitting and receiving the necessary electrical control signals forproper power flow control and coordination aboard the vehicle 10. Thecontroller 11 may include such additional control modules andcapabilities as might be necessary to execute the required power flowcontrol functionality aboard vehicle 10 in the desired manner.

Still referring to FIG. 1, the controller 11 may include amicroprocessor or central processing unit, read only memory (ROM),random access memory (RAM), electrically-erasable programmable read onlymemory (EEPROM), high speed clock, analog-to-digital (A/D) anddigital-to-analog (D/A) converter circuitry, and input/output circuitryand devices (I/O), as well as appropriate signal conditioning and buffercircuitry. Any algorithms and reference tables resident in thecontroller 11 or accessible thereby, including any algorithms orreference tables needed for executing the present method 100, can bestored in memory 18, i.e., non-transitory and tangible computer-readablemedia, and automatically executed by the hardware components of thecontroller 11, e.g., a host machine, in order to provide the respectivefunctionality.

Memory 18 used by the controller 11 may include any non-transitorymedium that participates in providing computer-readable data or processinstructions. Such a medium may take many forms, including, but notlimited to, non-volatile media and volatile media. Non-volatile mediamay include, for example, optical or magnetic disks, flash memory, andother persistent memory. Volatile media may include, for example,dynamic random access memory (DRAM), which may constitute a main memory.Such instructions may be transmitted by one or more transmission media,including coaxial cables, copper wire and fiber optics, including thewires that comprise a system bus coupled to a processor of a computer.Memory 18 may also include a floppy disk, a flexible disk, hard disk,magnetic tape, any other magnetic medium, a CD-ROM, DVD, any otheroptical medium, etc.

As noted above, the controller 11 provides proportional-integral controlfunctionality and state space observation capabilities. Both terms arewell understood in the art. The state space observer qualities of thecontroller 11 include the capability of modeling a physical system,e.g., clutch states or other desired states of the powertrain or of thetransmission 14 shown in FIG. 1, to provide an estimate of the internalstate of the system using various input and output measurements, as wellas state variables which are related by first order differentialequations. Inputs to the controller 11 may include measured enginespeed, speeds of the traction motors 16, 116, and an actual or estimatedoutput speed of the transmission 14. The controller 11 outputs a totaltorque command 21 to a given rotating component being controlled, e.g.,the traction motors 16, 116, to control both the speed of the member andthe level of driveline damping provided by that member.

Referring to FIG. 2, the common state space feedback portion of thecontroller 11 may be used to generate a closed-loop proportional torquecommand 52 for the damping control torques applied to a selected one ofthe traction motors 16, 116 of FIG. 1. As understood in the art,open-loop motor torque commands are subject to various constraints,e.g., battery power limits and/or motor torque limits. However,closed-loop motor torque commands are not considered in suchconstraints. As a result, the closed-loop proportional torque command 52could violate the constraints at times. The method 100 is thus appliedto address this situation.

When speed control is required, a frequency splitting routine 50 (seeFIG. 1), potentially including using a calibrated low-pass filter, isexecuted by the controller 11 to automatically split the closed-loopproportional torque command 52, i.e., the total proportional term of theproportional-integral control law provided by the controller 11, intodifferent frequency components. These include a high-frequency torquecomponent 56 used for the damping torque and a low-frequency torquecomponent 58 used for the speed control torque. Under a battery powerconstraint, a motor torque constraint, a clutch torque constraint, orother predetermined constraint(s), the low-frequency torque component 58can be selectively assigned a higher priority than the high-frequencycomponent 56 in order to maintain a required input speed to thetransmission 14 shown in FIG. 1, as well as clutch slip speed controlwithin the transmission 14.

The frequency splitting routine 50 can isolate the low-frequencycomponent 58 by passing the closed-loop motor torque command 52 througha suitable low-pass filter 60. Unless temporarily disabled, e.g., by asoftware trigger 51 or other selectively enabled signal, thelow-frequency torque component 58 is then fed to a computational node54.

The low-frequency torque component 58 bleeds down to a zero value whenthe low-pass filter 60 is reset or otherwise signaled by the softwaretrigger 51. The effect of the bleed down process is to shift thelow-frequency portion of the proportional torque command 52 from thelow-frequency torque component 58 to the high-frequency torque component56 in a blended fashion. In other words, all proportional torque, onceit has been fully bled to zero, goes to the high-frequency torquecomponent 56, and forms all of the proportional torque command 52. Whenthis happens, the low-frequency torque component 58 is equal to 0 Nm andthe integral torque component 63, i.e., the speed control torques, arealso equal to 0 Nm, thus making the speed control torque 64 equal to 0Nm.

At node 54, the low-frequency torque component 58 is subtracted from theclosed-loop proportional torque command 52 to calculate thehigh-frequency torque command 56. The low-frequency torque component 58is then sent to a computational node 62, where it is combined with theintegral torque component 63, i.e., the speed control torques. Theresultant torque command 64 is passed to a torque determination module66 of the control system 11. In this manner, a selected rotating memberof vehicle 10, e.g., the input member 17 of the transmission 14 shown inFIG. 1, still receives the low-frequency torque component 58 through acommon speed control path whenever the predetermined constraint ispresent. When the constraint is no longer present, the high-frequencytorque component 56 may be provided as well.

These two torque values can be independently limited downstream of node54 as needed. This allows the input speed to the transmission 14 of FIG.1 to be maintained, as well as clutch slip speed control, for examplewhen the vehicle 10 of FIG. 1 is running against motor torque, clutchtorque, and/or battery power constraints. In other words, the speedcontrol torques, which are generally but not necessarily limited to theintegral control term and low-frequency proportional control term, canbe allowed to take priority as needed over the high-frequencyproportional control terms so as not to violate the constraint.

Referring to FIG. 3, the present method 100 begins with step 102,wherein the control system 11 shown in FIG. 1 generates the totalproportional torque command 52 of FIG. 2. At step 104, the controller 11determines whether speed control is not required for a particularoperating state. If speed control is not required, the controller 11executes step 106. However, step 108 is executed if speed control isrequired.

Speed control may be required whenever there is at least one speeddegree of freedom. As used herein, the term “speed degree of freedom”refers to the number of speeds that can be independently controlled.With speed control, up to two speeds can be controlled at a given time,e.g., clutch input speed and clutch slip, or two clutch slips when in aneutral state (two speed degrees of freedom). One speed degree offreedom is present in a mode state, e.g., input speed only. There may beno controlled speeds at all in a fixed gear case, i.e., zero degrees offreedom, as the speeds are dictated by the vehicle. Therefore, in oneembodiment speed control is determined as being required in a neutralstate and any mode state, and not required in a fixed gear state.

At step 106, the proportional torque command 52 is not split, and thefull closed-loop proportional torque provides driveline damping asneeded. Since torque splitting is effectively disabled, the low-passfilter 60 of FIG. 2 is bled down so that all of the proportional torquegoes to the high-pass side as noted above with reference to FIG. 2. Forexample, the optional software trigger 51 shown in FIG. 2 may be trippedwhen a vehicle condition is present in which speed control is notrequired, with an input to the low-pass filter 60 of FIG. 2 set to zerowhen this condition occurs. A filter coefficient of the low-pass filter60 can then be set to a relatively fast response to expedite the bleedoff process.

Other conditions or modes may also exist where one traction motor isused for speed control, and the other traction motor is not. In otherwords, the speed degree of freedom affects the one motor, while theother motor's speed is dictated by another speed. This is normally thecase where the motor closest to the input is used for speed controlwhile the motor closest to the output is used for damping control.

At step 108, the controller 11 of FIG. 1 initiates the frequencysplitting routine 50 to thereby split the total proportional torquecommand 52 into the high-frequency torque component 56 and thelow-frequency torque component 58 shown in FIG. 2. These torque commandsare processed and applied as set forth above, i.e., with thehigh-frequency torque component 56 used to damp driveline oscillations,and the combined low-frequency torque component and integral torque usedto provide speed control as needed.

While the best modes for carrying out the invention have been describedin detail, those familiar with the art to which this invention relateswill recognize various alternative designs and embodiments forpracticing the invention within the scope of the appended claims.

The invention claimed is:
 1. A method for optimizing torque control in avehicle having a control system and a powertrain, wherein the powertrainincludes a clutch, an electric traction motor powered via a battery, anda rotating member, and wherein the control system is configured toexecute a proportional-integral (PI) control algorithm and a state spaceobserver providing a state space feedback control law, the methodcomprising: generating a closed-loop total proportional torque commandusing the state space feedback control law; generating a total integraltorque command by executing the PI control algorithm; selectivelysplitting the total proportional torque command into a high-frequencyproportional torque component and a low-frequency proportional torquecomponent, via a frequency splitting routine or a calibrated low-passfilter, when speed control of the rotating member is required;selectively assigning a higher priority to the low-frequencyproportional torque component than the high-frequency proportionaltorque component during a predetermined vehicle constraint, wherein thepredetermined vehicle constraint is a power constraint of the battery, atorque constraint of the clutch, or a torque constraint of the electrictraction motor; passing a total proportional torque from the controllerto the rotating member to provide driveline damping control when thespeed control of the rotating member is not required; and passing thehigh-frequency proportional torque component to the rotating member toprovide driveline damping control, and passing the low-frequency torquecomponent and the total integral torque command through a common speedcontrol path to the rotating member when the speed control of therotating member is required.
 2. The method of claim 1, whereinselectively splitting the total proportional torque command includespassing the total proportional torque command through the calibratedlow-pass filter to thereby isolate the low-frequency proportional torquecomponent.
 3. The method of claim 1, wherein selectively splitting thetotal proportional torque command includes calculating thehigh-frequency proportional torque component via the frequency splittingroutine by subtracting the low-frequency proportional torque componentfrom the total proportional torque command.
 4. The method of claim 1,further comprising: selectively resetting or bleeding to zero thelow-pass filter used for splitting the total proportional torque commandupon detecting the presence of a fixed gear state of the vehicle.
 5. Themethod of claim 1, further comprising: applying separate gain limitersto the high-frequency torque component and the low-frequency torquecomponent during the predetermined vehicle constraint.
 6. The method ofclaim 1, wherein the speed control of the rotating member is requiredduring the predetermined vehicle constraint, the method furthercomprising: temporarily limiting the driveline damping torque for onlyas long as the vehicle constraint remains present.
 7. A vehiclecomprising: a controller having a proportional-integral (PI) controlalgorithm and a state space observer providing a state space feedbackcontrol law; and a powertrain having a clutch, an electric fractionmotor powered via a battery, and a rotating member whose speed anddamping characteristics are controlled by the controller; wherein thecontroller is configured for: generating a total proportional torquecommand using the state space feedback control law of the state spaceobserver; generating a total integral torque command by executing the PIcontrol algorithm; selectively splitting the total proportional torquecommand into a high-frequency proportional torque component and alow-frequency proportional torque component via a frequency splittingroutine or a calibrated low-pass filter when speed control of therotating member is required; and when the speed control is required:selectively assigning a higher priority to the low-frequency componentthan the high-frequency component during a predetermined vehicleconstraint, wherein the predetermined vehicle constraint is one of apower constraint of the battery, a torque constraint of the clutch, anda torque constraint of the electric traction motor; passing thelow-frequency proportional torque component and the total integraltorque command to the rotating member through a common speed controlpath to thereby provide speed control over the rotating member; andpassing the high-frequency proportional torque component to the rotatingmember to provide driveline damping control via the rotating member; andwhen the speed control is not required: passing the total proportionalcontrol torque to the rotating member to provide driveline dampingcontrol.
 8. The vehicle of claim 7, wherein selectively splitting atotal proportional torque command includes using the calibrated low-passfilter, and wherein a filtering frequency of the low-pass filter isallowed to vary with an operating mode of the vehicle.
 9. The vehicle ofclaim 7, wherein the powertrain includes a transmission, and wherein therotating member is connected to the electric traction motor and is usedto control an input speed to the transmission.
 10. The vehicle of claim7, wherein the controller is configured for bleeding down thelow-frequency torque component to a zero value when the low-pass filteris reset via a software trigger.